In recent years, commercial presentation of high-sound quality music sources is spreading through the market, and customers' requirements for a higher sound quality are strong. In order to reproduce a high quality sound source with a high sound quality, balance connection of speakers or a head phone is required. In order to achieve the balance connection, two poles each for left and right speakers are required, so that at least four poles are required. In the case of providing a shield, five poles are required if a ground terminal is added.
In contrast, the customers also have a strong demand for carrying a reproduction apparatus that reproduces the high-quality sound source with a high sound quality. In order to apply the balance connection to a mobile high-quality sound reproduction apparatus, a multipole plug, specifically, a plug having specifically at least four poles is required.
Multipole plugs of the related art are formed by providing a center pin formed into a shaft shape as a first pole, cylindrical sleeves arranged on an outer periphery of the center pin as a second pole and so forth, and an insulation layer formed by injecting a resin between the respective poles. In the multipole plugs having such a structure, if the number of poles is four or five, the thickness of the sleeves of the respective poles is reduced, and the cross-sectional area thereof is also reduced, which may result in a high resistance value. The higher resistance value may become an obstruct for reproduction with high sound quality. In addition, since spaces between the sleeves of the respective poles are small, the resin cannot be injected well therebetween, which may result in an increased defect rate.
In view of such points, a technology in which terminal strips obtained by forming divided cylinders having a shape of a sleeve divided in a circumference direction for two or three poles, providing a ring at one end thereof in a longitudinal direction as a connecting end, and providing a bent portion at the other end thereof for positioning are disposed around a center pin is proposed (For example, PTL 1).
However, with this technology as well, injection of the resin between the terminal strips of the respective poles without leaving any unfilled space is still difficult, and another problem arises such that the length of the plug is increased due to the provision of the bent portion.